After a firefighter has arrived at, and begun working at, a fire site, the present location of that firefighter may be difficult to determine, minute-by-minute. The firefighter may be working outside an enflamed structure but be hidden by the firefighting equipment or some other structure or by the local terrain. If the firefighter is working inside the structure, the problem of locating this person is doubly difficult, because line-of-sight location is usually impossible and because radio waves used for voice communication may not be transmitted past the structures walls. Visually perceptible markings have been developed for firefighters' out garments, and methods have been developed for locating the perimeter of a fire. However, methods for determining the present location of a firefighter or other emergency worker at the site of an emergency, second-by-second, no matter where the worker may be located, have not appeared yet.
Tung discloses a retroreflective protective helmet having a plurality of retroreflective stripes thereon that can be seen in darkened areas, if illuminated by light, in U.S. Pat. No. 3,885,246. The helmet requires line-of-sight visibility before the helmet can be illuminated and the retroreflected light can be visually perceived. Another protective and retroreflective helmet, with the same limitations on visual perception, is disclosed in U.S. Pat. No. 4,008,949, issued to Luna.
Bingham, in U.S. Pat. No. 4,533,592, discloses an upper body garment made of thermally stable, flame retardant material that includes a plurality of light-reflecting stripes thereon, for use in firefighters' coats. As in the Tung and Luna patents, use of this garment to locate a firefighter requires line-of-sight illumination of the stripes.
In U.S. Pat. No. 4,347,501, Akerberg discloses a portable alarm system useful for notifying others that the alarm sender requires assistance. The alarm signal carries a unique code that allows a central receiver to identify the sender. The alarm signal is relayed from the sender to the central station by intermediate retransmitters, positioned in or near the room where the alarm device wearer is located, that transmit the alarm signal with a code indicating the last known location of the wearer. The alarm device wearer would occasionally update the alarm system's knowledge of his/her location by moving to another room in the structure. This system requires that a one or more alarm signal retransmitters be located in each room of the structure and that the retransmitter perform its intended functions under all circumstances. Where a firefighter responds to a tire, these conditions will not often be present.
An out-of-range personnel monitor and alarm, useful for convalescent home residents and other monitored persons, is disclosed in U.S. Pat. Nos. 4,593,273 and 4,675,656, issued to Narcise. The monitored person carries a transceiver that receives a first signal and compares the first signal strength against a selected threshold that corresponds to a maximum distance the monitored person can move away from the first signal transmitter. If the first signal strength is below the selected threshold, the transceiver transmits a second signal that is received by a monitoring station, advising that the monitored person has moved outside the permitted range. This system requires that the region within which the monitored person moves is reasonable homogeneous in attenuating electromagnetic signals, and that the first signal generator can be located near the center of the permitted region of movement for the monitored person.
Engler et al disclose use of a high temperature resistant, retroreflective material for marking a firefighter's helmet, in U.S. Pat. No. 5,160,655. The helmet marking material reflects light directed at the helmet back toward the light source so that a firefighter's present location can be determined if (1) the firefighter is within a line of sight from the light source and is not concealed within a building and (2) the ambient gaseous medium at the fire site is not so smoke-filled that the light incident on, or reflected from, the helmet marking material is absorbed by the gas.
Treddenick, in U.S. Pat. No. 5,192,500, discloses a firefighter safety badge, having indicia on a first badge face regarding the medical history of the badge user, and having indicia on a second badge face noting the anticipated location of the badge user on the fire site. The second indicia can be removed to expose a plurality of indicator strips that are sensitive to different toxic gases, such as chlorinated hydrocarbons. The badge is intended to be secured to a post or other structure near where the badge user is working. However, if the present location of the badge user changes and the second badge face indicia is not changed to reflect this change, the badge user cannot be located using this indicia.
A personal alarm security apparatus that is worn on an arbitrary part of a person's body is disclosed by Young in U.S. Pat. No. 5,196,825. Normally, the apparatus transmits a first signal that is interpreted as indicating that no threatening event has occurred or is occurring. If an emergency or threatening event occurs, a second signals is transmitted. A redundant third signal is transmitted at the time the second signal should be transmitted, in case the second signal is not transmitted for whatever reason. The system uses two receivers to obtain some information on the wearer's present location when a second signal is received.
Several U.S. patents disclose sensing the approximate perimeter of a tire, using infrared or similar means to sense temperature level differences or other characteristics that distinguish enflamed from non-enflamed areas. These patents include U.S. Pat. No. 5,160,842, issued to Johnson, and a sequence of U.S. patents issued earlier to Brown de Colstoun et al (U.S. Pat. Nos. 4,567,367, 4,893,026 and 5,049,756). However, none of these approaches appears to allow determination of the present location of a firefighter or other emergency service worker within an enflamed region or other emergency site.
FM subcarrier signals and AM carder signals have been used for some types of radio wave communications. In U.S. Pat. No. 3,889,264, Fletcher discloses a vehicle location system in which the unsynchronized AM carrier signals from three or more AM radio stations form hyperbolic isophase grid lines that are used to determine location of a vehicle. The vehicle must be equipped with a three-channel, tunable receiver, and its location must be referenced to an initial known location by counting the number of isophase lines crossed after the vehicle leaves the initial location. Isophase drift is compensated for by subtraction from the count.
Dalabakis et al, in U.S. Pat. No. 4,054,880, disclose a radio navigation and vehicle location system employing three low frequency subcarrier signals received from three radio stations at a three-channel, tunable receiver located on the vehicle. Isophase lines crossed are counted after the vehicle leaves an initial known location. This system, like the Fletcher system, is a delta-position system that determines vehicle location only relative to an initially known location.
U.S. Pat. No. 4,646,290, issued to Hills, discloses use of F.C.C.-approved Subsidiary Communication Authorization (SCA) FM subcarrier signals for one way transmission. This patent discloses transmission of a plurality of messages, which may be delivered to the transmitter at a wide range of bit rates, to be transmitted at a single bit rate that is at least as large as the highest bit rate for message delivery. This method allows for downstream insertion of additional data.
An integrated radio location and communication system for a mobile station is disclosed by Martinez in U.S. Pat. No. 4,651,156. Each mobile station carries a transceiver that issues radio signals that are received by two or more signal transceiver reference sites having fixed, known locations. The transceivers at the mobile station and the reference stations are continuously phase locked to the RF carrier signal from a nearby commercial radio station. The radio station and the mobile station each transmit a brief, distinguishable range tone at a known sequence of times, and the range tone from each station is received by each reference station. From an analysis of the differences in arrival times of the range tones received from the radio station and from the mobile station, the reference stations determine the two-dimensional location of the mobile station. The mobile station uses the beat signal between two RF subcarrier frequencies to generate its range tone signal and to distinguish that mobile station transmissions from the transmissions of any other mobile station.
Young et al, in U.S. Pat. No. 4,660,193, discloses use of two SCA FM subcarrier signals, the first being amplitude modulated and the second being phase modulated, to provide a digital data transmission system. A subcarrier signal within this system may also be modulated to carry audio signals.
A multichannel FM subcarrier broadcast system that provides a sequence of relatively closely spaced channels, using independent sidebands of suppressed carriers, is disclosed by Karr et al in U.S. Pat. No. 4,782,531. The sideband signals are generated in pairs and are phase shifted before transmission. Upon receipt of the transmitted signals, the process is reversed. An earlier patent, U.S. Pat. No. 3,518,376, issued to Caymen and Walker, discloses a similar approach without use of signal phase shifting of pairs of sideband signals.
In U.S. Pat. No. 4,799,062, Sanderford et al disclose a radio location method that uses a central processing station, a plurality of signal repeater base stations with fixed, known locations, and a mobile station with a known location at any time. The central station transmits a master grid synchronization pulse, which serves as a time reference, to the other stations at a selected sequence of times. A roving station with unknown location transmits a pulse that is received by three or more base stations and is retransmitted to the central station. The central station determines the location of the roving station using the differences in time of arrival at each base station of the pulse transmitted by the roving station. The mobile station also transmits a pulse from time to time, and its known location is compared with its computed location by the central station to determine any multipath compensation required to reconcile the known and computed locations of the mobile station. The multipath compensation for a mobile station adjacent to the roving station is applied to correct the computed location of the roving station.
Ma, in U.S. Pat. No. 4,816,769, discloses receipt of SCA FM subcarrier signals for digital data paging at a radio receiver. The system measures signal-to-noise ratio of an output amplitude of a Costas loop, used to phase lock to the FM subcarrier frequency, to determine if the signal is sufficiently strong to be processed.
A system for detection of radio wave propagation time, disclosed by Ichiyoshi in U.S. Pat. No. 4,914,735, uses detection of phase differences for transmission of the signal over M (.gtoreq.2) different known signal paths to a target receiver. The transmitted signal includes a subcarrier signal, having a frequency that is higher than the transmitter clock frequency, modulated with a known modulation signal. The receiver has M demodulators for the signals received by the M different paths and has a phase comparator to compare the computed phases for each of these received signals. The phase differences are proportional to the signal path length differences, if compensation for transmission line distortions is included.
U.S. Pat. No. 5,023,934, issued to Wheeless, discloses a system for communication of graphic data using radio subcarrier frequencies. The data are broadcast on a subcarrier channel and received by a radio receiver that is connected to a computer. The computer receives the subcarrier signals, displays the graphic data on a computer screen, and performs other functions, such as transmission error checking and modification of the displayed graphic data. The system is intended for weather data communication and display.
Westfall, in U.S. Pat. No. 5,073,784, discloses a system for location of a transmitter ("unknown") at large distances, using a large network of pairs of spaced apart radio wave receivers whose locations are known and whose relative phases are synchronized. A signal, broadcast by the unknown transmitter at less than HF frequencies, is received at different time and space points by pairs of receivers. Simple geometrical computations allow determination of the location of the unknown transmitter by comparing times of arrival of the transmitted signal.
U.S. Pat. No. 5,170,487, issued to Peek, discloses use of FM sub-carrier signals for a pager system for mobile users. A plurality of transmitters are used, each of which transmits an FM subcarrier signal or a carrier signal modulated with a chosen message signal, slightly offset in time. Each page-receiving unit is assigned a time slot, during which the receiving unit dials through the set of frequencies corresponding to the FM subcarrier and modulated-carrier signals to determine if a page message has been sent for that mobile user.
A system that allows determination of an absolute location of a vehicle is disclosed by Kelley et al in U.S. Pat. No. 5,173,710. FM subcarrier signals are received from three radio stations with known locations but unknown relative phases by signal processors at the vehicle and at a fixed station with known location relative to the three radio stations. The fixed station processor determines the relative phases of the three radio stations FM subcarrier signals and broadcasts this relative phase information to the vehicle. The vehicle processor receives this relative phase data and determines its absolute location, using the phases of the FM signals it senses at its own location.
Chon, in U.S. Pat. No. 5,193,213, discloses an FM broadcast band system for receipt of relatively high frequency FM subcarrier signals. A tunable high pass receiver first circuit receives the carrier and a tunable low pass second circuit receives the subcarrier signal. Each signal can then be separately processed.
A navigation and tracking system using differential LORAN-C or differential Decca signalling is disclosed by Duffett-Smith in U.S. Pat. No. 5,045,861. A reference station transmits a reference signal to a mobile station and to three or more local LORAN-C or Decca (fixed) stations having known locations relative to the reference station. The fixed stations retransmit the reference signal to the mobile station, where the phase received signal differences are compared to determine the location of the mobile station.
Most of these systems use a single communication system, rather than integrating two or more communication systems to provide location or navigation information for a mobile user. What is needed is an integrated location determination system for automatically or discretionarily determining the present location of a firefighter or other emergency service worker second-by-second at an emergency site, whether the worker presently works outside or inside a structure. Preferably, the system should accumulate and report on he time the worker spends in one or more selected sub-regions at the site. Preferably, the system should be at least partly portable, should work indoors or outdoors, and should provide estimates of location with inaccuracies no greater than ten meters, and more preferably no greater than one meter. Preferably, the system should allow a choice between location information provided by two or more location determination systems, based on a comparison of one or more parameters that measure signal robustness and/or signal quality or station location for the signals received and analyzed by each communication system.